Scabies (Sarcoptes scabiei var. hominis)
Scabies is a contagious ectoparasite skin condition caused by the mite Sarcoptes scabiei var. hominis (No relation to lice). In this condition, mites burrow into human skin and lay their eggs, which later hatch and grow into adults. The characteristic symptoms of this condition include superficial burrows, intense pruritus (itching) and secondary infection.
Classification: Sarcoptes scabiei var. hominis (CDC)
Genus and Species- Sarcoptes scabiei, variation hominis
Mite, Itch Mite, Mange, Crusted Scabies, Norwegian Scabies, Sarcoptes scabiei , The Itch, Seven-Year Itch
**Scabies is occasionally referred to as “lice.” However, there is no clinical relationship between scabies and lice.**
History of Discovery
Scabies is an ancient disease. Based on archeological evidence from Egypt and the Middle East, scabies is estimated to date back over 2,500 years (Markell & Voge). The first recorded reference to scabies is believed to be from the Bible (Leviticus, the third book of Moses) ca. 1200 BCE. Later, the Roman philosopher Aristotle reported on “lice” that would “escape from little pimples if they are pricked” in the fourth century BCE (Roncalli); scholars believe this was actually a reference to scabies.
Nevertheless, it was the Roman physician Celsus who is credited with designating the term “scabies” to the disease and describing its characteristic features (Roncalli). The parasitic etiology of scabies was later documented by the Italian physician Giovanni Cosimo Bonomo (1663-1969 ADE) in his famous 1687 letter, “Observations concerning the fleshworms of the human body” (Roncalli). With this (disputed) discovering, scabies became one of the first diseases with a known cause (Markell & Voge).
Bonomo’s Drawings of the Scabies Mite
Clinical Presentation in Humans
The characteristic symptoms of a scabies infection include superficial burrows, intense pruritus (itching), a generalized rash and secondary infection. Acropustulosis, or blisters and pustules on the palms and soles of the feet, are characteristic symptoms of scabies in infants (DermNet).
The superficial burrows appear as short, S-shaped tracks in the skin, and are often accompanied by small, insect-type bites called nodules that may look like pimples (DermNet). These burrows and nodules are often located in the crevasses of the body, such as between fingers, toes, buttocks, elbows, waist area, genital area, and under the breasts in women (DermNet).
Burrows (arrows point to mites)
The intense itching and rash characteristic of scabies infection is caused by an allergic reaction of the body to the burrowing of the microscopic scabies mites. The rash can be found over much of the body; the associated itching is often most prevalent at night (CDC).
Secondary infection is often due to impetigo, a type of bacterial skin infection, after scratching. Cellulites may also occur, resulting in localized swelling, redness and fever (DermNet).
In immunocompromised, malnourished, elderly or institutionalized individuals, infestation can cause a more severe form of scabies known as crusted scabies or Norwegian scabies. This syndrome is characterized by a scaly rash, slight itching and thickened crusts of skin containing thousands of mites (CDC). Norwegian scabies is the form of scabies that is hardest to treat.
In individuals never before exposed to scabies, the onset of clinical signs and symptoms is 4-6 weeks after infestation; in previously exposed individuals, onset can be as soon as 1-4 days after infestation.
Mode of Transmission
The majority of scabies cases are transmitted by skin-to-skin contact with persons carrying the scabies mite. Less often, scabies can be transmitted by sharing of clothes and bedding. Theoretically, touching an object that a mite is on is a third mode of transmission; however, this is not at all common.
Host Immune Response
When scabies mites burrow into the human skin, the eggs, mites and feces trigger a host immune response. Like an allergic reaction, this autonomic immune response results in a rash, itching, and occasionally a fever. However, this immune response does not occur until days to weeks after infection, due in large part to the fact that scabies mites are genetically encoded with immunological ‘weapons’ that prevent the host from responding to its presence (Burkhart).
Scabies has no non-human animal reservoir. However, mites can survive on fingernails, clothes, towels, bed linens and other household objects for up to three days.
There is no vector in the scabies lifecycle. Scabies is transmitted by human-to-human contact. Mites cannot survive longer than 3 days without a human host.
Upon infection, adult mites dig into the upper layers of human skin, creating burrows. Eggs are deposited into the burrows and hatch as larvae 3 to 4 days later. The larvae then excavate new burrows and mature in approximately 4 days (Markell & Voge). Once infected, the scabies lifecycle will continue until medication is used to treat the disease.
The associated skin disease characteristic of scabies develops due to delayed immune system hypersensitivity. The incubation period for this itching and rash is usually 2-6 weeks. However, in individuals with prior exposure to scabies, the incubation period is much shorter: as little as 1 to 4 days (Markell & Voge).
Adult scabies mites are spherical, eyeless mites with four pairs of legs (CDC). They can be recognized by their oval, ventrally flattened and dorsally convex tortoise-like body and multiple cuticular spines (Arlian). Females are 0.30 to .45 mm long and 0.25 to 0.35 mm wide, and males are just over half that size (CDC).
Artistic depiction of Sarcoptes scabiei morphology
Life Cycle of Scabies mites Sarcoptes scabiei var. hominis
The scabies mite Sarcoptes scabiei var. hominis goes through four stages in its lifecycle: egg, larva, nymph and adult.
Upon infesting a human host, 1) the adult female burrows into the skin, where she deposits 2-3 eggs per day. These oval eggs are 0.10mm to 0.15mm long and 2) hatch as larvae in 3-4 days. Upon hatching, 3) the 6-legged larvae migrate to the skin surface and then burrow into molting pouches (these are shorter and smaller than the adult burrows). After 3-4 days, the larvae molt, turning into 4) 8-legged nymphs. This form molts a second time into slightly larger nymphs, before a final molt into adult mites. Adult mites then mate when the male penetrates the molting pouch of the female 5). Mating occurs only once, as that one event leaves the female fertile for the rest of her life (1-2months). The impregnated female then leaves the molting pouch in search of a suitable location for a permanent burrow. Once a site is found, the female creates her characteristic S-shaped burrow, laying eggs in the process. The female will continue lengthening her burrow and laying eggs for the duration of her life (CDC).
Signs and symptoms of early scabies infestation mirror other skin diseases, including dermatitis, syphilis, allergic reactions, and other ectoparasites such as lice and fleas (Arlian). Nevertheless, most cases of scabies can be diagnosed by a description of the symptoms and an examination of the skin. As scabies is an ectopic parasite that only infects the skin, there is no blood test to diagnosis this disease. The only definitive way to diagnose scabies is through identification of a mite, its eggs, or its fecal pellets in the skin (often using microscopy) (Markell & Voge). Two common tests used for diagnosing scabies are skin scrapings and felt-tip marker test. In skin scraping, a drop of oil or saline is placed on top of the affected skin area. A scalpel is then used to scrape the area of tissue samples, and the material is examined until the microscope to check for mites or eggs. In the felt-tip marker test, a washable felt-tip marker is drawn across the rash, followed by an alcohol wipe. This procedure helps identify burrows because the ink penetrates deeply into the skin.
Both tests have rather low sensitivity, as mites are often hard to find. So even if a test is negative, the medical provider may still recommend treatment.
Management and Therapy
Scabies is not curable without prescription medication. The two medication options are a prescription cream or prescription pills; the cream is by far the most common treatment.
The topical medication is a 5% Permethrin cream*, such as Elimite. 10% Crotamiton (Eurax) cream is suggested for infants less than 2 months of age (Markell & Voge). Creams should be applied to clean, dry skin from the top of the head to the bottom of the feet, with special attention paid to skin folds and the webs of the digits (between the fingers and toes). The topical cream is left on the skin for 10-14 hours, and then washed off in the shower. It is best to apply the cream at bedtime, and then wash it off in the morning.
*Other topical medications include 1% Gamma Benzene Hexachloride (Lindane) and 6% Sulfur. Lindane is older and less safe than other options. It has been suggested to cause neurotoxicity, especially in kids. Lindane is not recommended. Sulfur ointment is effective, but may require extra applications, is messy, smells bad, and stains clothing (Markell & Voge).
The prescription pill option is the antiparasitic drug Ivermectin. The recommended prescription is a single oral dose of 150-200 micrograms of Ivermectin per kilogram of body weight (see your medical provider for specifics). This option should not be used by small children or women who are pregnant or breast feeding.
The intense pruritus (itching) characteristic of scabies can be treated with antihistamines such as Diphenhydramine (Benadryl), Hydroxyzine (Atarax), Cetirizine (Zyrtec) and Promethazine (Phenergan). The itching and rash may last for up to two weeks after treatment for scabies.
Secondary infection by scratching of the skin occasionally leads to bacterial infections. These infections can be treated with oral antibiotics or antibiotic ointment.
For patients with crusted scabies, several applications of lotions, use of Ivermectin pills, and extensive skin care are required for management.
If scabies symptoms persist two weeks after initial treatment, treatment may need to be repeated.
Scabies is impressively democratic in its epidemiology: mites are distributed around the world, affecting all ages, races and socioeconomic classes in all different climates (CDC). However, it is more often seen in crowded and unhygienic living conditions (Green). Globally, there is an estimated incidence of 300 million cases of scabies a year, 1 million of which occur in the United States (Markell & Voge).
Public Health and Prevention Strategies/Vaccines
There is no vaccine available for scabies, nor are there any proven causative risk factors. Therefore, most strategies focus on preventing re-infection. To prevent re-infection, all household members and sexual partners of affected individuals should be examined thoroughly for scabies. In addition, it is important that all clothes, towels, bed linens and other household objects used in the last 3 days are washed and dried on high heat. Objects that are not machine-washable should be bagged and stored for a week (as mites cannot survive for more than 3 days off the body). Fingernails should be cut and cleaned thoroughly to remove any mites and eggs. Rugs, furniture, bedding, car interior, etc. should be thoroughly vacuumed, and then the vacuum-cleaner bag needs to be thrown away.
For people still infected with scabies, they should avoid scratching (to prevent secondary infection) and keep any open sores clean.
Useful Web Links
DermNet NZ: http://www.dermnetnz.org/arthropods/pdf/scabies-dermnetnz.pdf
CDC Scabies Info: http://www.dpd.cdc.gov/dpdx/HTML/Scabies.htm
Epidemiology of Scabies: http://epirev.oxfordjournals.org/cgi/reprint/11/1/126
Arlian, L. Biology, Host Relations and Epidemiology of Sarcoptes Scabiei. Ann. Rev. Entomol 1989; 34:139-61. 5 Feb. 2009 <http://arjournals.annualreviews.org/doi/pdf/10.1146/annurev.en.34.010189.001035?cookieSet=1>
Burkhart, CG. Recent immunologic considerations regarding the itch and treatment of scabies. Dermatology Online Journal; 12(7): 7. 13 Feb. 2009 < http://dermatology.cdlib.org/127/commentary/scabies/burkhart.html>.
Green, M. Epidemiology of Scabies. Epidemiological Reviews 1989; 11:126-150. 5 Feb. 2009 <http://epirev.oxfordjournals.org/cgi/reprint/11/1/126>.
Markell EK, John DT, Krotoski WA. Markell and Voge’s Medical Parasitology, 9th ed. Philadelphia: W.B. Saunders, 2006.
Roncalli, RA. The History of Scabies in Veterinary and Human Medicine from Biblical to Modern Times. Veterinary Parasitology 1987; 25: 193-198.
"Scabies." 5 Dec. 2008. Laboratory Identification of Parasites of Public Health Concern. DPDx. CDC, Atlanta. 5 Feb. 2009 <http://www.dpd.cdc.gov/dpdx/HTML/Scabies.htm>.
“Scabies.” 15 February 2008. DermNet NZ. New Zealand Dermatological Society Incorporated. 12 Feb. 2009 < http://www.dermnetnz.org/arthropods/pdf/scabies-dermnetnz.pdf>.